1. Field of the Invention
This invention relates to a non-aqueous electrolyte battery having a positive electrode, a negative electrode and a non-aqueous electrolyte interposed between the positive and negative electrodes.
2. Description of the Prior Art
Recently, in keeping pace with rapid progress in a variety of electronic equipments, investigations are going on in the field of a re-chargeable secondary battery as a battery that can be used conveniently for long and economically. Typical of the secondary batteries are lead storage batteries, alkali storage batteries and lithium secondary batteries. Of these, lithium secondary batteries are superior secondary batteries having such advantages as high output or high energy density.
These lithium secondary batteries are made up of reversibly introducing and desorbing lithium ions, a separator arranged between the positive and negative electrodes and a non-aqueous electrolyte. In general, lainar electrically conductive high molecular materials, carbon materials or metal oxides, doped with metal lithium, lithium alloys or lithium, are used as negative electrode active materials.
On the other hand, metal oxides, metal sulfides or polymers are used as the positive electrode active material. For example, non-lithium compounds, such as TiS2, MoS2, NbSe2 or V2O5, or lithium-containing complex materials, such as LiMO2, where M=Co, Ni, Mn or Fe, have been proposed. These compounds may also be used in combination.
As the non-aqueous electrolytes, a solution obtained on dissolving lithium salts in non-protonic organic solvents, such as propylene carbonate, is used.
As a separator, a high-molecular film, such as a polypropylene film, is used. The separator needs to be as thin as possible in view of lithium ion conductivity and energy density. The separator thickness is usually not larger than approximately 50 xcexcm in view of practical utility.
Although the tendency is towards a higher capacity of the lithium secondary battery, the battery material selection in search for the best material is underway for cost reduction. Although a spinel structure manganese oxide is well-known as a manganese oxide used for the positive electrode, the theoretical capacity of the spinel manganese oxide is of the order of 150 mAh/g which is below 274 mAh/g as the theoretical capacity of LiCoO2. For this reason, researches for a complex oxides represented by LiMnO2, having the theoretical capacity of the same order of magnitude as LiCoO2, are going on briskly.
Depending on the temperature at the the of synthesis, high temperature type LiMnO2 is reported (R. Hoppe, G. Brachtel and M. Jansen in Z. Anorg. Allg. Chemie, 417, 1 (1975)), and low temperature type LiMnO2 is also reported (T. Ohzuku, A. Ueda and T. Hirai (Chem. Express. 7,193 (1992), as the complex oxides represented by LiMnO2.
Whilst the theoretical capacity is of the order of 300 mAh/g for both high temperature type LiMnO2 and low temperature type LiMnO2, if these LiMnO2 compounds are used in an actual non-aqueous electrolyte battery, the actual capacity is smaller than the theoretical capacity, because of limitations as to the potential region in which the electrolyte may remain in stability.
For example, the charging capacity in an actual non-aqueous electrolyte battery is said to be of the order of 150 mAh/g and 200 mAh/g for the high temperature type LiMnO2 and for the low temperature type LiMnO2, respectively. As for the discharging capacity, it is as low as 50 mAh/g or less, for the potential area not lower than 1.5 V, because the LiMnO2 compound is unavoidably modified in structure during the charging/discharging process.
Although the high temperature type LiMnO2 and the low temperature type LiMnO2 differ in the charging capacity, if the compounds are oxidized to a potential in the vicinity of 4.5 V, the discharging capacity becomes extremely small. Under these circumstances, it is desired to increase the discharging capacity of the LiMnO2 compound.
It is therefore an object of the present invention to provide a non-aqueous electrolyte battery having an improved capacity of the lithium manganese oxide used for the positive electrode.
The present invention provides a non-aqueous electrolyte battery including a positive electrode containing a complex oxide of a transition metal, a negative electrode arranged facing the positive electrode and containing metal lithium, lithium alloy, or a carbon material capable of doping and undoping lithium, and a non-aqueous electrolyte interposed between the positive electrode and the negative electrode. The complex oxide of a transition metal is a complex oxide of lithium and manganese represented by the general formula LiMn1xe2x88x92yByO2, with 0 less than y less than 1.
In the present non-aqueous electrolyte battery, according to the present invention, in which LiMn1xe2x88x92yByO2 obtained on adding B to LiMnO2 is used as a positive electrode, LiMnO2 is stabilized to realize a large capacity.
That is, according to the present invention, in which a lithium manganese oxide, part of Mn of which is replaced by B (LiMn1xe2x88x92yByO2), is used as an active material for the positive electrode, it is possible to realize a non-aqueous electrolyte battery having a large capacity.